Interferons (IFNs) and cytotoxic lymphocyte (CTL)-induced death are major pathogenic factors underlying injury in cutaneous lupus (CLE) and dermatomyositis (DM), but the extent of, and manner in which these pathways interact remains unclear. This dual PI program will define pathogenic mechanisms in CLE and DM, with a view to improved precision in diagnosis, disease monitoring and therapy. Numerous gaps exist in the diagnosis and management of these diseases, including incomplete understanding of the components participating in amplification of immune-mediated tissue damage, and lack of precise probes and biomarkers for diagnosis, subclassification and prediction/monitoring response to therapy. The proposal is based on significant preliminary data, broad scientific and clinical expertise, and a well-established and growing resource of extensively defined clinical materials. Our recent studies have identified a distinct cutaneous phenotype (in seronegative DM patients) associated with autoantibodies to MDA5, an autoantigen regulated by type I IFNs and cleaved during lymphocyte-mediated cytotoxicity. Our preliminary studies show that there are novel type I and/or type II IFN-inducible autoantigens targeted in DM and SLE (such specificities are undetected in conventional antibody screens) and provide evidence for expression of type II-IFN-specific markers preferentially in a subset of CLE patients, suggesting that this pathway might be therapeutically tractable in some CLE patients. Additionally, we have recently defined novel keratinocyte-specific autoantigens recognized by antibodies from patients with DM/CLE, and have identified 1 to date: keratin-5, an immature type II keratin expressed in basal keratinocytes. Several IFN-induced and keratinocyte-specific autoantigens are modified during UVB- or CTL-induced cell death, placing these antigens at the hub of damage and repair pathways in interface dermatitis. The specific goals of this proposal are: (i) generate and validate innovative tools (novel autoantibodies recognizing IFN-induced or proliferative keratinocyte autoantigens, and markers of CTL- mediated cell death in the skin) to define and quantify pathogenic pathways active in DM and CLE, (ii) interrogate disease mechanisms in affected patient skin by defining the site(s) of novel autoantigen expression, and define whether CTL activity is focused on these cells and (iii) use novel precision markers of 3 distinct mechanistic pathways in DM/CLE to define which pathways change in response to new therapeutic intervention, and are associated with the most striking clinical effects. The proposed studies will provide powerful new tools to precisely define the activity of pathogenic pathways in specific target tissues in individual patients in vivo, thereby facilitating diagnosis, prediction nd monitoring of clinical course in autoimmune skin diseases. The studies will address whether disease subsets defined using these markers respond differently to newly introduced therapy, providing proof of concept that specific pathway markers in target tissue can be used for patient classification and selection of therapy.